To control one channel on the RGBW controller/dimmer remotely through a double momentary switch (using S2) on the Dimmer 2.
Solution, and some questions:
Install the RGBW unit and its load, add it to the HC2 and assign its outputs. In my case, all 4 are "input/output" to control separate monocolor LED strips.
Install the Dimmer 2 and its load and add it to the HC2
Select the master module of the Dimmer 2.
Go to Advanced - Associations